Honestly, there's no reason to restrict Nagash's friendcircle to 3, when in the past he always favored 9. And there are tons of great ideas just sitting around for expansion of the Mortarch's ranks:
Old Characters:
* as mentioned, Khalida and Isabella. Both survived to the very end, both are interesting characters who saw drastic changes in their dynamics during the End Times that would provide a compelling starting point for new directions in their characterization moving forward.
* Krell, and old favorite. Not a ton of personality, but that's not a mark against him, it's kind of the point of his archetype, and he always played it well. I'd love to see a new Krell model mounted on a dread abyssal.
* Dieter. The End Times saw his transformation from a human necromancer/liche into a supernatural being managing the flow of souls in the underworld for Nagash so that the big guy could focus on mortal affairs. It would be interesting to bring him back in a sort of Charon to Nagash's Hades sort of role. He wouldn't even need an in game presence, he could work as just a background character, managing the day to day duties of a god of death for Nagash. Maybe play him off as loyal and dedicated, but somewhat exasperated by his master's focus on mortal affairs to the detriment of managing the afterlife. That said, while a backseat role in the fluff would work for him, he could also work on table as a massive incorporeal spectral figure, sort of like Sauron in his guise as the Necromancer in lord of the rings lore. You could play him up as the direct commander of Nagash's Nighthaunt forces, and tie that into his role managing the flow of souls.
New Characters:
* that Stormcast guy Nagash captured. The first corrupted Stormcast would make for a great mortarch, and a powerful symbol re-asserting Nagash's dominion over the dead. He could come in at the head of an entire corrupted undead stormcast minifaction.
* that 'first ghoul' guy hinted at in the Flesh Eaters battletome, whether it turns out to be Ushoran or someone else. A Ghoulish mortarch would fill an undead-type conceptual niche currently missing, and Nagash recapturing this guy and using him to assert dominance over the Flesh Eaters more generally could provide an interesting narrative arc.
Missing Conceptual Niches: So far we have two vampyric mortarchs and one skeleton/lich/necromancer type mortarch. But there are several classes of undead that it would be cool to see represented at the Mortarch level.
* as mentioned above, there's currently no Ghoulish mortarch
* a ghostly mortarch would be cool (maybe Dieter?)
* Arkhan is a skeletal Mortarch, but we don't have one that's explicitly a wight. Maybe bring back Krell?
* The morghasts are compelling units that really set the look for GW's end times undead, it would be great to see them expanded with heroes, named characters, and maybe a mortarch of their own.
* Zombies are pretty bottom rung, but if the deadwalker faction gets expanded with some higher ranked creature types (flesh golems, maybe?), I could see a rotting fleshy mortarch being an interesting possibility
* Arkhan and Mannfred are pretty sorcerous already, but there's certainly room for a more specifically mortal necromancer type, perhaps heading up an expanded deathmages subfaction. Maybe use the opportunity to describe an in-setting college or academy of necromancers serving nagash that attracts (or abducts) prospective students from across all the various planes? Maybe something that practiced openly before the grand alliance of the gods collapsed, and has become a cursed legend in the time since?
And then there's Nagash's current narrative push to try and re-assert his dominion over the various non-death-but-still-basically-undead factions, any of which could see a Death faction counterpart, potentially led by their own Mortarch
* as mentioned above, undead stormcasts are the most obvious
* maybe death-faction corrupted sylvaneth spirits/haunted woods?
* death faction corrupted chaos warriors?
* maybe death-faction elves stolen from the elven gods as they wrest those souls back from Slaanesh? I'd frankly love to see the elves in general split out into separate alliances rather than all holed up in the already overcrowded order subfaction. Perhaps the newly reborn war-aspect Alarielle might find Sigmar's ways too restrictive and unnatural, and take her Sylvaneth elves to Destruction? Perhaps Malekith, or whatever he's called now, might reappear to try and take the elven souls Tyrion and Teclis free from Slaanesh, only to be killed by the brothers and cast into the underworld, where Malekith could strike a bargain with Nagash to return as a mortarch, and the elven souls he stole before being cut down could return as a new semi-undead shadow-elf subfaction?
The Death alliance is so anemic at this point that there's really nothing but room for expansion, both in terms of subfactions and in terms of named characters, Mortarchs or otherwise.
